This one really depends on the time taken by the system to connect
localhost non-blocking.
If it's immediate (that happens), the test is not relevant and this is
what the above is saying.
So this message may or not be printed on different runs.

testsockets : SUCCESS
testsockopt : Line 84: TCP isn't corkable

Is TCP_CORK or TCP_NOPUSH defined on Solaris (in "netinet/tcp.h")?

nope.
$ grep -E "TCP_CORK|TCP_NOPUSH" /usr/include/netinet/tcp.h

Same Solaris version for 1.6.x and 1.5.x tests?

yep.
Lemme dig around here a bit. Something is fishy about these results.
dc